1. Field of the Invention
The present invention relates to a magnetic disk drive and to a read channel IC used in the drive or, in particular, to a magnetic disk drive comprising a detector of defects in a disk medium caused by thermal asperity, a missing signal and an extra signal and relates to a read channel IC used for the detector.
2. Description of the Related Art
With the recent improvement in processor capacity and the wide-spread use of multimedia software, demand for a higher speed and a larger capacity magnetic disk drive, constituting a storage device for a computer, has been increasing. In order to improve the recording density of the magnetic disk, the current trend is toward the use of a MR head including a Magneto Resistive (MR) element. On the other hand, a PR4ML (maximum likelihood detection system of partial response class 4) system has come to be employed as a recording/reproduction system. As a result of the increased recording density of the magnetic disk, the flying height of the head has steadily decreased. Also, the rotational speed of the disk medium has increased to meet the requirement for a high data transfer rate. In fact, a magnetic disk having a rotational speed of more than 10,000 rpm has already been introduced.
In the case where a MR head is used with a magnetic disk drive, a phenomenon called thermal asperity occurs in which the medium comes into contact with the head so that the temperature of the MR element changes suddenly to thereby change the resistance value of the MR element. The thermal asperity offsets the head output in a DC fashion, saturates an automatic gain controller (AGC) of a demodulator circuit and makes it impossible to retrieve the required waveform. As a result, the amount of information which cannot be normally demodulated from a medium exceeds the correction ability of an ECC (error correcting code) circuit, thereby leading to the problem of a read error that cannot be corrected.
Some thermal asperities which make the read operation impossible at normal temperature last only a short length of time and can be corrected by ECC. However, other thermal asperities which make the read operation impossible persist for a long time due to the increased height of medium protrusions at high temperatures or due to age deterioration. In the case where a medium defect is detected and registered during a test after manufacture of a magnetic disk drive, therefore, the position of a normal medium defect and the position of a defect due to thermal asperity are required to be registered separately from each other. Further, the recent trend toward a higher data transfer rate has made it impossible to detect minute defects in the medium which have conventionally been detected by extracting the output of the head IC out of the magnetic disk drive. In view of this, a new medium defect detector is required in place of the conventional one.
In the prior art, a defect position has been determined by changing the data to be written in the magnetic disk and the write position thereof and by judging whether a read error occurs when the data is read by the head. Also, the analog read signal that has been read out through the head from the magnetic disk medium is converted into a digital signal and compared with a variable slice level. The portion which exceeds the slice level is detected as a defective portion of the disk medium (Unexaminated Patent Publication (Kokai) No. 2-44272).
In the first method of determining a defect position digitally by judging whether a read error occurs or not, however, at least 108 write/read operations are required to determine a defect position on a medium constituting the magnetic disk. It is also difficult to set the proper number of the read/write operations to be executed to detect a medium defect. Another problem is that the type of medium defect, even if detected, cannot be identified.
The medium defects include a "missing" type and an "extra" type in addition to the thermal asperity described above. The "missing" is a defect in which the read signal assumes so low a crest value below a slice level that it cannot be reproduced. The "extra" is a defect in which the read signal, after falling once, rises again due to a flaw on the medium and crosses the slice level twice.
In the method of detecting a medium defect disclosed in Unexaminated Patent Publication (Kokai) No. 2-44272, on the other hand, the slice level is raised or lowered in order to determine the pulse position of the read signal subjected to analog-to-digital conversion. This method of medium defect detection, however, poses the problem in that it is impossible to detect that the read signal is offset in DC fashion in one direction due to a thermal asperity.